Field of the Invention
The present invention relates, in general, to an intake-air amount detecting system for an internal combustion engine which detects the amount of the intake-air for the internal combustion engine provided with an electronically controlled fuel injection device.
By maintaining an air-to-fuel ratio of gas mixture supplied to the internal combustion engine to a theoretical air-to-fuel ratio which permits an optimum performance of the engine, not only the operational performance of the engine is enhanced but also the cleaning ability of an engine exhaust gas cleaning device, particularly a catalyst type exhaust gas cleaning device, is optimized. For this reason, it is necessary to accurately detect the amount of the intake-air by weight to the engine to supply an amount of fuel to the engine, which amount depends on the amount of the intake-air. In the past, in the detection device of this type, the volumetric amount of the intake-air has been detected indirectly from the r.p.m. of the engine and the intake negative pressure or from the r.p.m. of the engine and the aperture of a throttle valve. As a result, due to manufacturing tolerance of the engine, deterioration of engine performance, misadjustment of clearances for intake valve and exhaust valve, and variation in time of the performance of an air cleaner, large errors have been included in the measured amount. In addition, since the amount measured was in a volumetric flow rate, a complex apparatus has been required, resulting in increase of cost.
Another prior art device is the so-called heat wire anemometer disclosed in the U.S. Pat. No. 3,747,577, which is shown in FIG. 1. There is provided in an intake tube a temperature dependent resistor 18, which, together with other three resistors 19, 20, and 21, constitute a bridge circuit so that the resistor 18 is kept at a constant temperature and hence the resistance value of the resistor 18 is kept at a constant value. The resistor 18 is cooled by the intake-air. As the flow rate of the intake-air increases, the resistor 18 is considerably cooled. In order to maintain the balance in the bridge circuit, the amount of current flowing through the resistor 18 is increased to increase the amount of heat generation by the resistor 18. When the flow rate of the intake-air decreases, the current through the resistor 18 is reduced to reduce the heat generation by the resistor 18 in order to maintain the balance in the bridge circuit. The flow rate of the intake-air can be determined by detecting the amount of current through the resistor 18. In this case, however, if the temperature of the intake-air per se changes, the relation between the flow rate of the intake-air and the current is also changed. To avoid such inconvenience, the U.S. Pat. No. 3,747,577 uses a temperature measuring resistor 30 instead of the resistor 20 in the bridge circuit, as shown in FIG. 2, to measure the temperature of the intake-air for necessary compensation. In this system, the air flow rate is given by the following formula; EQU R.sup.. i.sup.2 = (a + b .sqroot.U) (T - To)
where R is a resistance value of the resistor 18, i is an amount of current through the resistor 18, a and b are constants, U is a flow rate of the intake-air, T is a surface temperature of the resistor 18 and To is a temperature of the intake-air. As seen from the above formula, since the air flow rate U is calculated as a four order function of the current i, i.e. F(i.sup.4), it is necessary that the current i should be accurately detected. Furthermore, although the amount of the intake-air is calculated from the air flow rate in the prior art system, it is the amount of air measured directly in the flow rate by weight, as described above, that is required in the fuel injection device which uses the intake-air amount detection device of the above type.